Taking titanium further

At its recent Titanium Technology Days event, Rorschacherberg, Switzerland-based milling specialist StarragHeckert reiterated its commitment to a titanium future with the introduction of several new machines, expanding its capability for the manufacture of aerospace components. Simon Lott reports. Speaking at the opening of StarragHeckert's customer event last June, Group CEO Dr Frank Brinken described the company's mantra as ‘precisely productive'. While its products are by no means exclusive to the aerospace industry, the company prides itself on its strength in titanium milling and as such, a large amount of product development is focused around the industry, especially in expanding the parameters for the milling of large and complex components.

“If you look at the main trends in the aerospace industry one thing is absolutely clear - Six Sigma compliance,” explains Brinken. “We have seen increasing titanium content first in military applications but now also in civil aircraft and we are also seeing a move to larger complex 5-axis integrated structures and a definite globalisation of the supply chain. In particular, countries that are aiming for a significant increase in air travel will also increase their share in the aerospace supply chain.

“Six Sigma is definitely a game changer for the aero industry, but how do you build a reliable, compliant machine? The solution for machine performance is quality through core machine design, both of the company's latest products to hit the market; the SPC (Super Precision Centre) 7120, the first conventional mill to be produced under the SIP (Société d'Instruments de Precision) brand; and the huge BTP 5000/2 5-axis profiler exhibit typical attention to detail.

Best of both worlds

The SPC 7120 is the first in a new line of products to come from the rejuvenated SIP brand, which was purchased by the StarragHeckert group in 2006. Best known for its long history of ‘ultra precision' jig boring machines, the SPC range is very much a cross between its own machine building expertise and StarragHeckert's milling know-how, and is specifically designed to deliver minimal compromise between precision and speed. Thus, where SIP's traditional range can achieve accuracies of under 2.5µm and typical Starrag machines are around the 6-7µm mark, SPC machines can achieve 4µm from its conventional mill design. However, a variable 50kW main drive motor allowing for speeds of 15,000rpm means that working feed and rapid traverse rates of 40m/min in X, Y and Z can be achieved for efficiency in volume production.

“We have seen in the last 4-5 years that even in the aerospace sector, particularly civil, customers are looking for lower cost manufacturing by use of increased productivity and are frequently questioning whether they require such high tolerances on certain parts,” adds SIP managing director Jean-Daniel Isoz. “That trend is quite new compared to the automotive industry but it has emerged, so we need to be able to offer this functionality.”

With production taking place at SIP's temperature controlled Geneva facility, each machine undergoes around 200 hours of hand scraping with the base, linear guideways, ballscrews, spindle heads and axes all treated, offering as close to perfect geometry as possible for long-term precision.

There has also been extensive design consideration given to thermal management throughout the machine, with the use of additional thermal shields to improve isolation from heat sources, temperature and elongation sensors in key components, active coolant temperature compensation and controlled cooling of the direct drive rotary table, spindle gearbox and ballscrews. A thermosymmetrically designed bed ensures that the weight and mass of components each side is closely matched allowing for even expansion across the machine and an air stream barrier helps to prevent build-up of air between the floor and the machine interior.

There are currently two variants in the SPC range– the 4-axis 7120 and 7140 models, with the main difference being capacity. The 7120 has a machining volume of 1,200mm x 900mm x 1,200mm in X, Y and Z whereas the 7140 offers a larger 1,450mm x 1,100mm x 1,300mm. 5-axis versions are to be launched at this month's IMTS exhibition in Chicago.

Building a beast

The other major piece of product news at the event was the new horizontal BTP (Big Titanium Profiler) 5000/2, one of the largest machines ever to be produced by the company and available with one or two fully independent spindles, effectively doubling capacity for long, thin aerospace parts such as stringers, engine pylons and control surfaces. Weighing in at 200 tonnes, the machine has spent four years in development and over a year to build, requiring 15 trucks to deliver. The machine can deliver strokes in the single spindle version of 5,000-6,000mm in X, 2,000mm in Y and 1,200mm in Z, with ±100˚ in A and 360˚ rollover in B.

Although StarragHeckert typically offers both gear and motor spindles for its machines to meet individual requirements, the BTP range uses only gear spindles for several important reasons. “While the purchase cost is identical,” expands Brinken, “overhaul on a gear spindle is usually $40,000-$60,000 whereas for a motor spindle it can be $20,000k-$300,000 because the bearings are very expensive. Motor spindles also have to go back to the manufacturer and can take several weeks to repair. More significantly though, there is an EU eco design directive coming for machine tools. We are not sure when exactly but it will be in the next few years, basically meaning that high electrical consumption components will no longer be accepted.”

Key features of the machine are the 37kW gear driven spindle developing 940Nm of torque and achieving up to 8,000rpm spindle speed, and a steel on steel worm drive A-axis with constant gear driven torque. Hydraulically supported twin ballscrews on all guideways yielding maximum stiffness and minimising wear, sturdy machine structure and heavy-duty drive motors have all been designed to ensure vibrations are minimised, maximising tool life and allowing for maximum chip removal. Automation has also been a prime consideration, with a double pallet changer available, capable of lifting up to 18 tonnes unbalanced.

Important also for machines of this size is thermal management, which has been extensively catered for. The bearings of the gearbox, power train spindle and vertical slide are all catered for by a compressor cooling circuit, the column and Z-bed using process coolant adapted to the machine temperature. A unique patented flexiture system ensures parallelism between the spindle and workpiece sides of the machine to dial out inaccuracies.

Having already designed and produced the first BTP 5000/2 machine for a major aerospace OEM, the company is convinced that the efficiency of the horizontal dual spindle design will be of real benefit in allowing larger and more ambitious component designs to enter into production.

www.starragheckert.com